Species and hybrids
In the current Plants of the World Online treatment, Rhipsalidopsis gaertneri and R. rosea are the two accepted species in the genus. Both originate from southern Brazil and grow as epiphytes. Their horticultural hybrid R. × graeseri has the parental formula R. gaertneri × R. rosea. Publications using Hatiora gaertneri, H. rosea and H. × graeseri refer to the same taxonomic entities [2-WWW].
Partner compatibility in sexual reproduction
Controlled crosses among 10 diploid Easter cactus cultivars identified nine self-incompatibility groups. The study included Rhipsalidopsis gaertneri, R. rosea and their hybrid R. × graeseri, then treated within Hatiora. Two cultivars shared one group, while each of the remaining eight represented a separate group. Compatible parental combinations produced fruit set ranging from 35 to 100% under the experimental conditions [Boyle 2003].
The compatibility pattern was consistent with gametophytic self-incompatibility controlled by a multiallelic S locus. From the number of groups, the author inferred that at least five S alleles occurred in the cultivar set. This result describes genetic diversity within the cultivated collection and explains the importance of a genetically compatible partner for seed production [Boyle 2003].
Flower opening and senescence
Easter cactus flowers change how widely they open during the day. In a comparison of eight Rhipsalidopsis cultivars, observations were made at midday to reduce the influence of these movements on developmental assessment. The researchers distinguished early, middle and late stages of the open flower, followed by senescence. They used perianth position, firmness and colour, together with the opening of stigma lobes. Such a developmental description allows more precise flower comparisons than simply counting days since a bud was noticed [Scott et al. 1994].
At 20 ± 1°C, relative humidity of 75 ± 5% and a twelve-hour photoperiod, mean flower longevity among the cultivars was 6.7–12.2 days. 10 flowers on at least four plants were assessed per cultivar. ‘Mira’ had the shortest longevity, while the others reached 9.6–12.2 days. Senescence was recognised by loss of firmness and perianth changes, often followed later by abscission of the whole flower. The experiment concerns selected cultivars whose parentage was unresolved [Scott et al. 1994].
Hormones and the capacity to regenerate shoots
In a comparison of Rhipsalidopsis and Schlumbergera cultures, successive subcultures, transfers of tissue to fresh medium, were associated with changes in hormone regulation. In the studied Rhipsalidopsis, increased regenerative capacity accompanied changes in auxin metabolism and enzyme activity involved in cytokinin turnover. The authors associated these changes with adventitious shoot formation [Sriskandarajah et al. 2006].
Callus of ‘CB5’ was also used in experiments introducing genes with Agrobacterium tumefaciens. Some material retained the capacity to regenerate shoots after prolonged laboratory selection. The authors identified ‘CB5’ as a hybrid of R. gaertneri and R. rosea. Results of that experiment are therefore presented in the R. × graeseri card, retaining the individual cultivar name [Al-Ramamneh et al. 2006a] [2-WWW].
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